The evolution of the tetrapod humerus: morphometrics, disparity, and evolutionary rates

Marcello Ruta, Jonathan Krieger, Kenneth Angielczyk, Matthew Wills

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The present study explores the macroevolutionary dynamics of shape changes in the humeri of all major grades and clades of early tetrapods and their fish-like forerunners. Coordinate point eigenshape analysis applied to humeral outlines in extensor view reveals that fish humeri are more disparate than those of most early tetrapod groups and significantly separate from the latter. Our findings indicate sustained changes in humeral shape in the deepest portions of the tetrapod stem group and certain portions of the crown. In the first half of sampled tetrapod history, subclades show larger than expected humeral disparity, suggesting rapid diffusion into morphospace. Later in tetrapod evolution, subclades occupy smaller and non-overlapping morphospace regions. This pattern may reflect in part increasing specialisations in later tetrapod lineages. Bayesian shifts in rates of evolutionary change are distributed discontinuously across the phylogeny, and most of them occur within rather than between major groups. Most shifts with the highest Bayesian posterior probabilities are observed in lepospondyls. Similarly, maximum likelihood analyses of shifts support marked rate accelerations in lepospondyls and in various subclades within that group. In other tetrapod groups, rates either tend to slow down or experience only small increases. Somewhat surprisingly, no shifts are concurrent with structural, functional, or ecological innovations in tetrapod evolution, including the origin of digits, the water-land transition and increasing terrestrialisation. Although counterintuitive, these results are consistent with a model of continual phenotypic innovation that, although decoupled from key evolutionary changes, is possibly triggered by niche segregation in divergent clades and grades of early tetrapods.

Original languageEnglish
Pages (from-to)351-369
Number of pages19
JournalEarth and Environmental Sciences Transactions of the Royal Society of Edinburgh
Issue number1-2
Early online date15 Oct 2018
Publication statusPublished - 1 Mar 2019


  • amniotes
  • eigenshape analysis
  • evolutionary rate shifts
  • humeral morphology
  • lepospondyls
  • lissamphibians
  • tetrapodomorphs

ASJC Scopus subject areas

  • General Environmental Science
  • General Earth and Planetary Sciences


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